Because he doesn't know if you have control or not, or other newbs may see you applying leglocks, they might get it into their heads to do the same thing. They might not know what the **** they are doing however, and could hurt someone even if you are safe with them.

When instructing a larger group of people, it is easier to just make blanket policies. No leglocks for whitebelts. Rather than telling some whitebelts they can and others they can't. This would just lead to confusion and resentment from those that can't and from raw newbies.

New guys can be dangerous at any rate because they don't know how to properly measure how much force they are applying against their partners. I know a guy, a BJJ purple, who got his arm broken showing a new student how to do a basic armbar. The guy got into position, closed his hips, slammed his arm down, and -SNAP-, it broke right there.

Ankles/knees are especially easy to damage, so I can see the need for some rules like that to be in place.

"This is why we are here. Because the Martial Arts for too long have been cloaked in an unnecessary level of secrecy bordering on mysticism, and its in these shadows that the cockroaches love to hide. -Phrost"

BJJers are fairly hardcore about their belts. I lost my white belt once, and when my instructor found out, he looked at me like I just sodomized a box full of kittens.

I have lost, approximately, 6 blue belts. This includes one that had "Not your fucking belt" and my name written in sharpie, in large letters.
BJJers aren't "hardcore" about belts, you just look like the sort of guy that would sodomize kittens.

Ankle locks, heel hooks, neck cranks, KNEEEBAAAAAAAHs... whatever. There's no difference between what a white belt and a purple belt is allowed to do where I train.

Never had a problem with it, never seen anybody injured doing it in training.

The only issue is when you hit tournaments. Some restrict white belt levels quite a bit, so if some guys gets a single leg, and you counter with a rolling KNEEEBAAAAAAH... you're the man and on top of the world... right until the ref points out you're disqualified.

you are on your instructor's mats. his mats. his rules. he knows what he's doing and what he's talking about, you don't, and that's why you're paying him to remedy this. if he says no leg locks for white belts, then LISTEN TO THE MAN.

I am quoting this throughout this site for a million years. Well said. Why don't people get this?